Vibronic transitions in the luminescence spectra of Pr3+ in Na5La(MoO4)4

JOURNAL OF

LUMINESCENCE ELSEVIER

Journal of Luminescence 60&61 (1994) 7477

Vibronic transitions in the luminescence spectra of Pr3 + in Na 5La(Mo04)4 C. de Mello Donega*, M.J.D. Crombag, A. Meijerink, G. Blasse Debre Institute, Unirersiti lJtrecht,

P.O.

Box 80 0003508 TA (Jirechi, The Netherlands

Abstract 3 ~ in Na Vibronic transitions have been investigated in the luminescence spectra of Pr 5La _~Pr~ (MoO4)4 (x ~ 3.5 x 102) at 4.2 K. Oneand two-phonon vibronic replicas are observed. The transition probability AvB 3 * in several isostructural host-lattices are discussed. Concentration enhancement for of the the vibronic lines transitions is observed of Pr in the excitation spectra only. This phenomenon is ascribed to superexchange interaction between Pr3~ions over distances of bA.

1. Introduction

of Pr3 + violate the M process selection rules, suggesting a contribution by the A process. In order to

The intraconflgurational 4f’~spectra of rare-earth (RE) ions often show weak vibronic features, des2 and pite the shielding of the 4f electrons bystrength the 5s can 5p6 outer shells. The vibronic coupling be determined by two contributions, namely the vibrationally induced forced electric-dipole transitions (M process) and the classical Franck—Condon replicas (A process) [1]. Vibronic transitions due to the M process obey selection rules, viz. AJ t~ 2 and coupling with i.r.-active vibrations only [1]. For a transition within the 4f” shell the A process is usually neglected, since the Huang—Rhys coupling factor is assumed to be zero. There is evidence, however, that this approximation is notpointed justifiedout(see we have previously [2],e.g. the [1,2]). vibronicAstransitions

*

Corresponding author,

check this assumption we have investigated the luminescence spectra of Pr3.c in Na 5La(Mo04)4, aiming at identifying two-phonon vibronic transitions, which are expected for the A process only. This host-lattice is particularly suited for our purposes, since its vibrational spectra show a distinct 400cm -‘ energy gap between the bending and the stretching vibrations of the MoO 4 2 group, in addition to a clear cutoff at 900cm - These characteristics make it possible to observe well-defined two-phonon replicas. Moreover, we compare the transition probabilities of vibronic transitions of 3 + in several isostructural (scheelite-like) hostPr lattices, namely LiYF 4,LaNbO4,Na5La(W04)4 35 and Na5La(Mo04)4. The influence concentration on the vibronic spectraofofthe Pr3Pr in Na 5La1_~Pr~(MoO4)4(x ~ 3.5x 102) was also investigated. In this paper we shall briefly discuss the relevant results. A more extended analysis will be presented elsewhere.

0022-23i3/94/$07.00 © 1994— Elsevier Science By. All rights reserved SSDI 0022-231 3(93)E0256-W

~.

C. de Mello Donegh el al.

/ Journal of Luminescence 60&61

2. Experimental Powder samples of composition Na5 La1 ~Pr~(MoO4)4 (x = 5 x i0~, 5 x i0~ and 3.5 x 102), La1_5Pr~NbO4 and Na 5La~ ~Pr5(WO4)4 (y = 5 x 10~) were prepared using solid state techniques. 3+ conventional was prepared as described in Ref. [2]. LiYF4: Pr The crystal structure was checked by X-ray powder 4’4’ or other optical impudiffraction. Absence of Pr rities was verified by diffuse reflectance spectroscopy. The optical instrumentation has been described in detail elsewhere [2]. The vibrational spectra of Na5La(Mo04)4 were of measured by the spectrochemical analysis group the University Utrecht.

3. Results and discussion 3~ We shall first discuss the influence of the Pr 3’4’. concentration the excitation vibronic transitions Figure 1 showsonthe spectra of of thePr 3P 0 3 + in Na emission of Pr 5La1 ~Pr~(MoO4)4 for two concentrations x at corresponding 4.2 K. The spectra consist of several sharp lines to transitions 2 configuration (3H~ .2 and within the 4f i 16) The superscript (1) indicates the lowest crystal3H field component of the 4 ground level, which is —*

______________________________________ _______________

(1994) 74—77

75

the only one significantly populated at 4.2 K. Intense vibronic sidebands are observed in addition to the zero-phonon lines. All lines are enhanced 3H~~ 3P relative to 34’ the concentration. 0 electronic line with inThe integrated intencreasing Pr sity ratio R of the vibronic 3H~~ 3P features and the zerophonon line of transition is takenThe as a measure of the vibronic 0coupling strength. 3H~ 3P ~16 lines above the 1, ~H~1 3P start at about 450cm 0 zero-phonon line, imposing 410cm 1 as an upper limit on the frequency of the vibronics included in the ratio R. The estimated values of the ratio x =x 5= x 3.5 iO~,0.55 3 Rare and 0.14 0.85 forfor x 10- 2 for x = 5 x i0 clearly indicating the concentration enhancement of the vibronic excitation transitions. Vibronic transitions were also observed in the emission spectrum. The emission spectrum of Pr3 + in Na 5La(MoO4)4 at 4.2 K consists of several sharp lines the region 3P 490—740 to the in transitions 3Hnm, corresponding 3F 0 4,56, 234 and iD 3H 34’ ion. In contrast to the 4 of the Pr vibronic transitions in the excitation spectrum, the vibronic emission transitions do not intensity depend onratio the 3 + concentration: the vibronic Pr R for the 3P 3F 0 2 transition is 0.1 for all concentrations investigated. —~

—*

—~

-

—*

-

—*

—~

—+

Such concentration enhancement 3effects + can on be the atvibronic excitation transitions of Pr tributed to an increase of the degree of covalency induced by superexchange interaction between Pr3 + ions via the molybdate group. The influence of the degree of covalency on the vibronic coupling strength will be discussed below. The enhancement of the 3H 3P 4 2, ~16electronic transitions can be interpreted as an example of hypersensitivity —~

I

___________

~1’0

/ /i

420

-~

440

4è0

.~

v 480

(AJ = 2), which can be ascribed to an increase of the opposite-parity configuration admixing induced by the interaction. 3H 3PWe assume that the enhancement of the 4 1 lines is due to the rise of the background by the enhancement of the 3H 4 ~I lines. The absence of concentration enhancement for the vibronic transitions in emission implies that the interaction is effective only when the ions are both in the same state. The probability —~

5 0

Wavelength (nm)

3P 3F Fig. 1. Intraconfigurational 3 ‘~ in Na excitation spectra of the 0—* 2 emission of Pr 5La i - Pr (MoO4)4 at 4.2 K, for x = 5 x l0~ (solid line) and x = 3.5 x 10-2 (dashed 3H~’—* line) 3p The spectra are normalized to the intensity of the 0 zerophonon line. V indicates vibronic sidebands.

—*

.

.

3+

that ions are both in the 3P two neighbouring Pr 0 excited state is evidently negligible. Further evidence for this model has been presented before

76

C’. tie Me/b Donegd

Ct

al. / Journal of Luminescence 60&61 (1994) 74 77

[3]. The largest distance over which the interaction has observable consequences, r~,is estimated to be ioA by using the method described in Ref. [3]. Now we turn to the influence of the host-lattice on the transition probability AviB for the vibronic 3H 3P 3’. The value of of Pr AvJB4--~ can 0be transitions estimated from the decay time of the 3P 0 level, the ratio R, and the emission spectrum after discounting iD2 emission lines. A detailed description of this method has been given before [2]. The estimated values of AvIB and other relevant data are3’4’ summarized in Table 1. The concenis 0.1 mol% for LiYF tration of Pr 4: Pr and 0.05 mol% for the remaining lattices. Table 1 shows that AviB increases with the shift of the zerophonon line and of the optical absorption edge of the lattice to lower energies. These shifts can be ascribed to two related effects: the increase of the covalency and of the polarisability of the ligands. This observation is consistent with the previously reported host-lattice dependence 3~[2]. of the intensity of vibronic transitions of Pr The vibronic part of the 3H 4 2 excitation 3P 3F and 0 —s 2 emission transitions were investigated under high-resolution. There is a good agreement between the positions of the vibronic lines in emission and excitation and the vibrational frequencies of the host-lattice. Vibronic coupling occurs directly modes involving the 3” not ionsonly butwith alsovibrations with vibrational within Pr molybdate group (bending and stretching). The the latter case is an example of cooperative vibronics (see e.g. [1]). The assignment of the vibrational —*

modes was made by comparison with the vibrational data of isostructural molybdates and tungstates [4]. The intensities of the vibronic lines do not appear to be determined by the Raman or i.r.-active character of the phonons which couple to the electronic transition. Figure 1 shows that the ratio R is the same for the 3H 3P 4 —s 0 (A.J = 4) and 3H 3P the 4 2 (AJ = 2) excitation transitions. As argued in the introduction the facts mentioned above are violations to the M process selection rules. We will discuss below new evidence for the significance3H~ of A process vibronics. Figure 2 shows 3P one of the 2 vibronic transitions, located at 800cm from the zero-phonon origin. Under magnification a vibronic line can be observed at l620cm’ from the zero-phonon line. Since the vibrational spectra of the host-lattice shows a clear cut-off above 900cm these two vibronic features are ascribed to one- and two-phonon replicas due to coupling with the molybdate stretching vibration (—~800cm - i) Two-phonon replicas are only expected for A processvibronic vibronics. For —+

—*

-

~,

a A process vibronic involving n phonons the ratio of the integrated vibronic intensity to the integrated zero-phonon intensity is given by [1] -

‘vib,n

=

e

S

(I)

n!

where S is the Huang—Rhys factor. From Eq. (I) and the ratio between the one-phonon replica and

xlOO T.i~i

a e I Vibronic transition probabilities AViB and the zero-phonon line position (ZP) for the Pr3* 3H~1—~ 3P 0 excitation transition in several host-lattices. The optical absorption edge (OAE) of the lattice is also included. All measurements were performed at 4.2 K. Spectral position (cm

>.

/ //

— I)

/ ‘..

Lattice

(s

1)

ZP

,

,‘

OAF

LiYF4 100 20873 80000 LaNbO4 1000 20534 40000 Na~La(WO4)4 1200 20530 42000 Na~La(MoO4)4 4000 20463 36000 ________________________________________________________

410

4~0

4~0 440 (nm) 3H~-. 3P Fig. 2. One- and two-phonon3* in Na,La(Mo0 2 vibronic replicas in the excitation spectrum of Pr 4)4 at 4.2 K. Wavelength

C. de Me/b Donegd el al. / Journal of Luminescence 60&61 (1994) 74—77

the zero-phonon line we estimate S to be 0.03. The ratio ‘vib 2/Ivib. is thus equal to S/2 or 0.015, which is only slightly larger than the experimental value of 0.01. This result strongly suggests that the A pro3+ cess contribution to the vibronic coupling of Pr is significant.

77

(Switzerland) for measurements on Na5La(W04)4: Pr. C.M. Donegá gratefully acknowledges the award of a scholarship by CNPq (Conseiho Nacional de Desenvolvimento Cientifico e Tecnológico Brazil). —

References Acknowledgements The authors are indebted to Prof. J.H. van der Maas and Mr. E.Th.G. Lutz (University Utrecht) for measuring the vibrational spectra of Na 5 La(Mo04)4, to HF. Folkerts (UU) for measurements on LaNbO4 : Pr, and to S. Schenker

[I] G. Blasse, mt. Rev. Phys. Chem. 11(1992) 71. [2] C. de Mello Donegá, A. MeiJerink and G. Blasse, J. Phys.: Condens. Matter 4 (1992) 8889. [3] C. de Mello Donegá, A. Ellens, A. Meijerink and G. Blasse, J. Phys. Chem. Solids 54 (1993) 293 [4] P. Tarte and M. Liegeois-Duyckaerts, Spectrochimica Ada A 28 (1972) 2029.